The invention relates to a process for the denitrification of bypass exhaust gases in a plant for producing cement clinker, where the plant has, in the gas flow direction, a rotary tube furnace for sintering of the cement clinker, and where the rotary tube furnace is connected via a rotary tube furnace inlet chamber to a calciner for deacidification of raw meal or to a rotary tube furnace riser shaft, and where the bypass exhaust gas is taken off in the region of the rotary tube furnace inlet chamber.
To produce cement, silicate- (SiO44−) and carbonate-containing (CO32−) rock is milled, formally freed of carbon dioxide (CO2) by deacidification of the carbonate (CO32−) to the oxide (—O) (in the form of quicklime, CaO) at elevated temperature and, in a subsequent step, the mixture composed of silicate-containing (SiO44−) and carbonate-containing (CO32−) raw meal is sintered at elevated temperature in a rotary tube furnace to give cement clinker (a mixed crystal composed of calcium silicates of various stoichiometries). Sintering takes place at comparatively high temperatures which range up to 1450° C. To generate these high temperatures in a rotary tube furnace, burner flames which have a temperature of from 1800° C. to up to 2000° C. are necessary in the rotary tube furnace. At these high temperatures, nitrogen oxides (NOx) are formed by combustion of the nitrogen (N2) present in atmospheric air. Apart from the combustion of atmospheric nitrogen (N2), a further nitrogen oxide source may be present in the fuel used for generating heat in the rotary tube furnace. This is generally present as chemically bound nitrogen (R—N), for example as amines (R—NH2) in organic secondary fuels. Nitrogen oxides (NOx) disproportionate in the presence of moisture from the atmospheric air to form various oxoacids of nitrogen having different stoichiometries. These oxoacids of nitrogen are the cause of undesirable acid rain which, in large quantities, undesirably decreases the pH of forest soils and soils of agricultural land and thereby very greatly decreases the resistance of trees and plants to diseases. Note therefore has to be taken of legal obligations for avoiding nitrogen oxide emissions, and these are becoming ever stricter. This means that the absolute emissions always have to be reduced further.
It is not only the emission of nitrogen oxides (NOx) which is an undesirable problem in the production of cement. Apart from nitrogen oxide emission, it is also necessary to keep the cement clinker free of halides (F−, Cl−, I−) and sulfates (SO42−). The halides (F−, Cl−, I−), in particular the chlorides (Cl−), and the sulfates (SO42−) of the alkali metals (Li, Na, K) and alkaline earth metals (Mg, Ca) have a strong adverse effect on the properties of the cement to be produced from the cement clinker. The alkali metal halides (LiF, LiCl, LiI, NaF, NaCl, NaI, KF, KCl, KI) and alkaline earth metal halides (MgF2, MgCl2, MgI2, CaF2, CaCl2, CaI2) and alkali metal and alkaline earth metal sulfates (Li2SO4, Na2SO4, K2SO4, MgSO4, CaSO4) cause condensation problems in the rotary furnace inlet chamber. The alkali metal halides (LiF, LiCl, LiI, NaF, NaCl, NaI, KF, KCl, KI) and alkaline earth metal halides (MgF2, MgCl2, MgI2, CaF2, CaCl2, CaI2) and alkali metal and alkaline earth metal sulfates (Li2SO4, Na2SO4, K2SO4, MgSO4, CaSO4) go over into the gas space at appropriately high temperatures in the rotary tube furnace, which firstly exercises a desirable purifying effect on the cement clinker during the sintering thereof. When the hot burner gases go over into a calciner or into a rotary tube furnace riser shaft, where the temperature of the rotary tube furnace exhaust gas decreases abruptly as a result of the transfer of the heat present in the rotary tube furnace exhaust gas to the endothermic deacidification reaction of the carbonate (CO32−) to form quicklime (CaO), the alkali metal halides (LiF, LiCl, LiI, NaF, NaCl, NaI, KF, KCl, KI) and alkaline earth metal halides (MgF2, MgCl2, MgI2, CaF2, CaCl2, CaI2) and alkali metal and alkali earth metal sulfates (Li2SO4, Na2SO4, K2SO4, MgSO4, CaSO4) condense and, on further cooling, solidify and produce caked material on the walls of the rotary tube furnace inlet chamber, which can ultimately block the plant completely.
To remove the alkali metal halides (LiF, LiCl, LiI, NaF, NaCl, NaI, KF, KCl, KI) and alkaline earth metal halides (MgF2, MgCl2, MgI2, CaF2, CaCl2, CaI2) and alkali metal and alkaline earth metal sulfates (Li2SO4, Na2SO4, K2SO4, MgSO4, CaSO4) from the process, DE 197 18 259 A2 teaches taking off a gas substream as bypass exhaust gas from the rotary tube furnace inlet chamber and separate off the alkali metal halides (LiF, LiCl, LiI, NaF, NaCl, NaI, KF, KCl, KI) and alkaline earth metal halides (MgF2, MgCl2, MgI2, CaF2, CaCl2, CaI2) and alkali metal and alkaline earth metal sulfates (Li2SO4, Na2SO4, K2SO4, MgSO4, CaSO4) in a separator. The remaining gas is, after being completely freed of dust, released into the free atmosphere. It is precisely these gases which still contain a high proportion of nitrogen oxides (NOx). As plants for producing cement are becoming ever larger, the amounts of gas and the amounts emitted are becoming so great that emission into the free atmosphere is no longer tolerable.
A third emission which has to be avoided is the emission of undesirable chlorinated and polychlorinated dibenzodioxins, often referred to colloquially and in simplified form as “dioxins,” accompanying the dust-free exhaust air from the bypass exhaust gases, and the emission of undesirable chlorinated and polychlorinated dibenzofurans, often referred to colloquially and in simplified form as “furans.”
Dioxins and furans are formed spontaneously in combustion processes in the presence of carbon (C), halogens (F2, Cl2, I2) and oxygen (O2). When secondary fuels whose burning behavior is difficult to control are used, the combustion can take place to only an unsatisfactory extent with, for example, halogen-containing solvents also forming halogen oxides in the event of incomplete combustion and these oxides decomposing at the temperature of the rotary tube furnace and thereby allowing free halogen (Cl2) to be formed. However, owing to the very high temperatures in a rotary tube furnace, dioxin and furan formation is not to be expected in the rotary tube furnace, so that rotary tube furnace exhaust gases and bypass exhaust gases should be free of dioxins and furans. Nevertheless, the electrostatic dust filters which are frequently used for removing dust from bypass exhaust gases sometimes tend to generate electric arcs because of the high static charge and can in the presence of halogens (F2, Cl2, I2) produce not only carbon compounds and oxygen (O2) but also dioxins and furans; halogens (F2, Cl2, I2), especially chlorine (Cl2), originate, as mentioned above, from the combustion of secondary fuels or are introduced with the raw meal. The dioxins and the furans are formed at moderate temperatures in the dust precipitator from carbon monoxide (CO), carbon dioxide (CO2) present in the bypass exhaust gas, possibly from soot (Cx) and from chlorine (Cl2) in combination with atmospheric oxygen (O2). Dioxins and furans are formed, despite their comparatively complex structure, spontaneously under these conditions.
Thus, alkali metal halides (LiF, LiCl, LiI, NaF, NaCl, NaI, KF, KCl, KI) and alkaline earth metal halides (MgF2, MgCl2, MgI2, CaF2, CaCl2, CaI2) and alkali metal and alkaline earth metal sulfates (Li2SO4, Na2SO4, K2SO4, MgSO4, CaSO4), nitrogen oxides (NOx) and also dioxins and furans are formed in the process. These materials have to be removed from the process and should not go into the free atmosphere.
DE 101 58 968 A2 proposes blowing the bypass exhaust gases which possibly contain dioxins and furans as a result of dust removal as cooling air into the recuperation zone back into the recuperation region of a clinker cooler which follows the rotary tube furnace in order to quench the freshly sintered cement clinker. From there, the dioxin- and furan-containing bypass exhaust gases are blown back as secondary air into the rotary tube furnace in the process and the dioxins and furans are burned at the temperature of the rotary tube furnace. In the case of large plants for producing cement, the amounts of the bypass exhaust gases to be treated are so large that simple recirculation to the recuperation region of the clinker cooler leads to further problems. The recirculated bypass exhaust gases are low in oxygen as a result of the single or multiple passage through the circuit within the plant for producing cement. If the entire bypass exhaust gases were to be circulated, a low-oxygen and, as a result of combustion, carbon dioxide-rich atmosphere would be formed in the plant as a result of oxygen consumption due to combustion; this atmosphere is finally so low in oxygen and rich in carbon dioxide that it can no longer be employed as secondary air for the burner in the rotary tube furnace.
Avoidance of dioxins and furans by circulation of bypass exhaust gases thus leads to new problems which have to be overcome.
In addition, there are, in the prior art, processes for mixing the bypass exhaust gases with the exhaust gases from a heat exchanger which is located downstream of the calciner or downstream of the rotary tube furnace riser shaft in the gas flow direction. In this way, the concentrations of undesirable emissions permitted by law, here especially the concentration of nitrogen oxides (NOx), in the total exhaust gas from the plant for producing cement are reduced by dilution. However, this process does not lead to a reduction in the absolute amounts of the actual emissions. Furthermore, legislators are in future also expected to pass emission regulations which prohibit plant operators from diluting undesirable emissions.
Avoidance of nitrogen oxides, dioxins and furans by mixing the bypass exhaust gases with other gases given off from the process is thus also not an acceptable route.
One process which is known for denitrifying the total exhaust gases from a plant for producing cement is the SNCR (selective noncatalytic reduction) process. In this process, ammonia (NH3) in gaseous form, urea (CH4N2O) and/or an ammonium solution (NH4+) is injected at suitable places into the plant for producing cement. The ammonia (NH3), the urea (CH4N2O) and/or the ammonium solution (NH4+) is intended to react, at an appropriate residence time and in the correct temperature window, with the nitrogen oxides (NOx) in the exhaust air from the rotary tube furnace, which flows through the entire plant, and form nitrogen (N2) and water (H2O) from ammonia (NH3) and nitrogen oxides (NOx). As a suitable place, the top of a calciner has been selected for this purpose in the prior art. However, the high flow velocities and the large diameter of the plant (from 4 m to 8 m) and also the high loading of the gases conveyed through the plant with raw meal make quantitative reaction of the nitrogen oxides (NOx) difficult to control. The residence time and the flow, and also the interfering dust atmosphere, make denitrification by means of ammonia (NH3) in a plant for producing cement difficult to control. Either nitrogen oxide breakthrough (NOx), ammonia breakthrough (NH3) and/or nitrous oxide breakthrough (N2O), sometimes even isocyanic acid breakthrough (HNCO), is obtained and the total gases conveyed in the plant are denitrified by means of ammonia (NH3), urea (CH4N2O) or ammonium compounds (NH4+X−) for only an unsatisfactory part of the time. In the remaining time, the abovementioned breakthrough takes place. In addition, the combustion in the plant for producing cement is deliberately carried out stepwise, as a result of which a change from a chemically oxidative environment and a reductive environment occurs, forcing a reaction of carbon monoxide (CO) with the nitrogen oxides (NOx) to form nitrogen (N2) and carbon dioxide (CO2). The nitrogen oxides (NOx) are chemically reduced by means of carbon monoxide (CO). Denitrification by means of the SNCR process for total denitrification of the entire exhaust gases from the plant has thus hitherto not been successful to a satisfactory extent.
Denitrification of the total exhaust gases by means of the SNCR process is thus also not a feasible route.
Concentration on the denitrification of the rotary tube furnace exhaust gases which are loaded with high levels of nitrogen oxides (NOx) by means of the SNCR process is also not a feasible route because the required temperature window from about 950° C. to 1000° C. is not attained in the bypass exhaust gas. Although not inconsiderable conversion takes place at lower temperatures, quantitative chemical reduction of the nitrogen oxides does not take place below the temperature window from about 950° C. to 1000° C. Furthermore, the bypass exhaust gas has to be cooled strongly in order to condense out alkali metal halides (LiF, LiCl, LiI, NaF, NaCl, NaI, KF, KCl, KI) and alkaline earth metal halides (MgF2, MgCl2, MgI2, CaF2, CaCl2, CaI2) and alkali metal and alkaline earth metal sulfates (Li2SO4, Na2SO4, K2SO4, MgSO4, CaSO4). It would be conceivable to firstly denitrify the bypass exhaust gas by means of the SNCR process and then condense out the alkali metal halides (LiF, LiCl, LiI, NaF, NaCl, NaI, KF, KCl, KI) and alkaline earth metal halides (MgF2, MgCl2, MgI2, CaF2, CaCl2, CaI2) and alkali metal and alkaline earth metal sulfates (Li2SO4, Na2SO4, K2SO4, MgSO4, CaSO4). However, this would require heating of the bypass exhaust gases and require the process of denitrification to be carried out in the presence of the alkali metal halides (LiF, LiCl, LiI, NaF, NaCl, NaI, KF, KCl, KI) and alkaline earth metal halides (MgF2, MgCl2, MgI2, CaF2, CaCl2, CaI2) and alkali metal and alkaline earth sulfates (Li2SO4, Na2SO4, K2SO4, MgSO4, CaSO4) present in the gas space, which in turn leads to breakthrough of ammonia (NH3), nitrogen oxides (NOx) and/or nitrous oxide (N2O), and possibly isocyanic acid breakthrough (HNCO).
Even though the removal of alkali metal halides (LiF, LiCl, LiI, NaF, NaCl, NaI, KF, KCl, KI) and alkaline earth metal halides (MgF2, MgCl2, MgI2, CaF2, CaCl2, CaI2) and alkali metal and alkaline earth metal sulfates (Li2SO4, Na2SO4, K2SO4, MgSO4, CaSO4) from bypass exhaust gases represents a relatively small problem today, the associated necessity of denitrification and avoidance of the emission of dioxins and furans continues to be in need of improvement.
It is therefore an object of the invention to provide a process for the denitrification of bypass exhaust gases.